This application relates to a method and apparatus useful in detecting minute quantities of a target molecule in a sample.
Detection of chemical particulates, contaminants, or vapors has important applications in public safety, national security and the security industry for detecting explosives and illegal drugs. Presence of such explosives, drugs, or chemical agents usually leaves a small amount of particles on surfaces, in the air, water, or sediment where the contraband is stored or handled. For example, particles of explosives or drugs on surfaces of luggage or clothes at airports or public buildings may lead to discovery of an attempted bombing or drug smuggling.
Techniques based on positive or negative ion formation via charge transfer to a target molecule (i.e., explosive constituent), or electron capture under multi-collision conditions in a Maxwellian distribution of electron energies (with a peak at about 40 millielectron volts (meV)) at the source temperature (300 K) have been developed. Such techniques include atmospheric sampling, glow-discharge ionization (ASGDI), atmospheric pressure ionization (API), electron capture detection (ECD) and negative-ion chemical ionization (NICI).
In order to maximize ion formation, and therefore target molecule detection sensitivity, high electron currents at low energies (<10 meV) were needed at the point of attachment between electrons and trace target molecules. To provide a better “match” between the electron energy distribution function and the attachment cross section, the electron reversal technique was developed. In this technique, electrons are brought to a momentary halt by reversing their direction with electrostatic fields. At a reversal region R, the electrons have zero or near-zero energy. A beam comprising target molecules is introduced, and the zero or near-zero energy electrons are attached to the molecules of the beam. Slowing the electrons to subthermal (<10 meV) energies is required because the cross section for attachment of several large classes of molecules (including the explosives, chlorohalocarbon compounds and perfluorinated carbon compounds) is known to increase to values larger than 10−12 cm2 at near-zero electron energies. In fact, in the limit of zero energy, these cross sections are predicted to diverge as ε−1/2, where e is the electron energy.
This basic electron reversal technique has been improved upon to allow for better reversal geometry, higher electron currents, lower backgrounds and increased negative-ion extraction efficiency. See Bernius and Chutjian, U.S. Pat. No. 4,933,551; Boumsellek and Chutjian, U.S. Pat. No. 5,374,828, which are incorporated herein by reference.
In U.S. Pat. No. 4,933,551 ('551), the electron emitter was an indirectly heated oxide cathode with a planar (flat) surface geometry. The shapes of the emitter and mirror regions were used to calculate the fields-and-trajectories for the emitter region. The electron trajectories derived from this calculation were used to calculate the fields-and trajectories for the reversal region. Subsequently, the trajectories were examined graphically. The mirror design was accepted if the incoming and reversed trajectories appeared to overlie each other in the graph. However, graphical examination of electron trajectories limits determination of the electron kinetic energy to no better than 20 meV. Furthermore, calculation of the electron trajectories was limited to a single pass through the electron optics lens stack. Since the reversed currents were not accounted for in the electron source region there was an inherent inaccuracy in the calculated electron trajectories from the cathode. These trajectories are affected by the space charge of the reverse current, but this effect was not taken into account in the '551 patent.
In contrast to the '551 patent, the electron source disclosed in U.S. Pat. No. 5,374,828 ('828) included a small “shim” electrode in the cathode region which created a mismatch between electron source and reversal equipotential configurations. The '828 patent disclosed a system that blended the spherical equipotentials adjacent to the emitter into the planar equipotentials of the reversal region. In contrast, the system disclosed in the '551 patent utilized planar equipotential surfaces both adjacent to the planar emitter and in the reversal region. The “shim” electrode disclosed in the '828 patent compensated for the differences in initial geometries by converting the spherical equipotentials at the cathode into planar equipotentials suitable for reversal within the planar equipotential configuration disclosed in the '551 patent. Essentially the spherical trajectories were “straightened out” into a parallel electric-field configuration which is not as precise as maintaining spherical equipotentials throughout the system.
Accordingly, in order to increase target molecule detection sensitivity, there exists a need to maximize ion formation in a reversal region of a device employing the electron reversal technique.